1. Field of the Invention.
This invention is in the field of remote sensing and more specifically in the area of coherent laser radar system used for imaging, adaptive beam forming, and directed energy weapons.
2. Relevant Background.
Laser radar (ladar) systems are used to gather information about targets by sending a laser beam from a transmitter to the target, collecting light scattered from the target, and processing the received light to extract information. Hard-target ladar systems scatter light from natural or man-made targets and the information of interest for extraction includes: range, velocity, vibrations, shape, materials, and surface properties such as texture or color. Future military systems will rely heavily on the use of ladar systems to perform functions that include intelligence gathering, surveillance and reconnaissance imaging, target designation, communications and directed energy. There are, however, several difficulties in optimally performing these functions. One difficulty is that of overcoming atmospheric turbulence, which distorts optical wavefronts and hence degrades images. A second difficulty is that high-resolution imaging fundamentally requires large apertures, since the resolution of imaging systems is proportional to λ/D, where λ is the wavelength and D is the aperture diameter. For example, the aperture diameter required to produce a laser spot with 5 cm size (or an imaging system with spatial resolution of 5 cm) at λ=1.5 μm and a stand-off distance of 50 km is 1.5 m. Incorporating such a large aperture onto an aircraft or Unmanned Aerial Vehicle (UAV) is extremely difficult, not only because of the required surface area, but also because of the volume required to encompass the focal length of the optics, as well as any required sensors.
The problems associated with high-resolution imaging are well-known in astronomy, as well as in military systems, where atmospheric degradation limits the useable aperture of telescopes. In astronomy, passage of light from distant stars through the turbulent atmosphere to a terrestrial telescope results in severe degradation of the image quality if the coherence area at the telescope is substantially smaller than the physical light collection area. The coherence area, defined as the area over which the received light is considered to be spatially correlated, is in turn determined by refractive index variations in the optical path experienced by light beams propagating through different paths in the atmosphere, as well as by aberrations present in the optics. When the coherence area is significantly smaller than the physical aperture of the imaging system, the telescope can be viewed as performing an incoherent addition of multiple images from smaller telescopes. Since the spatial resolution of an imaging systems is directly proportional to the aperture diameter, such images will be degraded in relation to what they would be in the absence of atmospheric distortions. This very significant problem is currently addressed using adaptive optics based around wavefront sensors and deformable mirrors. For example, in an increasingly common astronomical adaptive optics system, a laser beam at 589 nm is used to illuminate sodium atoms high in the atmospheres. Light scattered from the sodium is collected with an adaptive optics system. By sensing the wavefront across the full aperture using Shack-Hartmann sensors, for example, and deforming the mirror appropriately to compensate for the local phase distortions, the one-way atmospheric effects can be compensated for and the image quality improved. Such systems work well in some circumstances but are of limited use for ladar systems. In particular, they do not address the issue of compactness. Astronomical telescopes are large but, since they are most frequently ground-based, there is generally not a strong constraint on dimensions. The situation is very different in airborne imaging scenarios where space is at a premium. Furthermore, astronomical adaptive optics systems rely on the presence of an artificially created beacon, such as the deliberately excited sodium emission, to perform the compensation.
It is furthermore recognized (see, e.g., the paper “Atmospheric compensation with a speckle beacon in strong scintillation conditions: directed energy and laser communication applications” by Weyrauch and Vorontsov, Applied Optics vol. 44, pp. 6388, 2005) that conventional methods using Shack-Hartmann sensors fail to work well under conditions of strong atmospheric turbulence.
A well-known non-astronomical system that deploys adaptive optics is the Airborne Laser (ABL) developed for missile defense applications. This highly complex system incorporates a high energy laser with the capability to lock onto and track targets. To do so effectively, as well as to concentrate the high energy laser pulse in a small area, the system incorporates wavefront sensing and deformable mirror technology to adjust the wavefront of the transmitted beam. The current lack of a compact and reliable method of implementing wavefront correction, especially over a significant field-of-view, is problematic. Improved solutions for ABL and other directed energy weapons are highly sought after.
Adaptive optics methods are also desired for subaperture co-phasing applications, such as when multiple mirrors are assembled to form a large telescope. U.S. Pat. No. 5,905,591 to Duncan et al. describes a method of adjusting the relative position of multiple small telescopes (subapertures) to create a large aperture area space borne telescope capable of producing high resolution. A disadvantage of the described method is that it uses actuators and phase sensing devices to maintain the relative position of all subapertures, which adds to the complexity of the structure.
U.S. Pat. No. 6,229,616 to Brosnan et al. describes a wavefront sensor capable of measuring the phase fronts across a large aperture without performing any corrections to an image. The described system has several disadvantages. First, the local oscillator is derived from an arbitrary point, as opposed to a point that has been selected for its desirable scattering properties, which generally makes it very weak. The local oscillator may be amplified before mixing with the signal. However, optical amplifiers add (often substantial amounts of) noise, which degrades the sensitivity of the system, and, further, this is an approach that may only work in very limited circumstances when the signal-to-noise ratio (SNR) is high. The method described may be useful for measuring wavefront amplitudes but does not disclose how that information can be used to produce high resolution imagery or how it can be utilized to transmit a laser beam to produce high on-target energy density. Additional differences between the Brosnan approach and that of the present invention will be clarified in the text that follows.
A further issue of considerable importance in imaging through turbulence is that of anisoplanatism. Turbulence is spatially inhomogeneous which means that light propagating from an extended target experiences varying degrees of distortions depending on which part of the turbulent region the light propagated through. As seen from the imaging sensor, the distortions are only approximately constant over a small angular range, referred to as the isoplanatic angle. The target size corresponding to the isoplanatic angle is referred to as the isoplanatic patch or isoplanatic diameter. Conventional adaptive optics using deformable mirrors can only correct for one isoplanatic angle and generally require complex multi-conjugate optics to perform corrections over a wide field of view (FOV).
Anisoplanatism is of importance not only in imaging of a target but also illumination with a high energy beam. Subject systems can be used for two near simultaneous purposes. One is the imaging function discussed above. The second is in transmitting a high energy laser beam aimed at damaging or destroying the target. In this case, an important parameter is delivering a maximum amount of energy density (energy per unit area) to the target, which means that the footprint of the illuminating beam should generally be minimized. This in turn means that the beam should be predistorted in such a way that an initially distorted beam becomes undistorted after propagation through the turbulent atmosphere to the target. Since the distortions vary with look angle, the system should be able to measure and apply predistortions over an extended range of angles.